1. Field of the Invention
Isomerization of sym-octahydrophenanthrene (s-OHP) to sym-octahydroanthracene (s-OHA) in the presence of AlCl.sub.3 catalyst is well known. Of particular interest herein are methods for promoting the rate of isomerization of s-OHP to s-OHA.
2. State of the Art
Anthracene is useful as a starting material in processes for making dyestuffs, antioxidants and medicinals. Anthracene and its isomer phenanthrene are both found in coal tar, with phenanthrene being about four times more abundant than anthracene. Since phenanthrene is more abundant than anthracene, much attention has been given to conversion of phenanthrene to anthracene.
The only practical conversion of phenanthrene to anthracene involves three steps. As a first step, phenanthrene is catalytically hydrogenated to sym-octahydrophenanthrene (s-OHP); secondly, s-OHP undergoes catalyzed isomerization to sym-octahydroanthracene (s-OHA); and thirdly, s-OHA is dehydrogenated to anthracene. In the second step of isomerization of s-OHP to s-OHA, in the presence of AlCl.sub.3 as the isomerization catalyst, known isomerization reactions provide relatively low yields of the desired s-OHA isomer, or relatively high yields of by-product impurities, or typically require relatively long s-OHP-to-catalyst contact times for formation of the s-OHA isomer.
For example, a 1924 German publication [G. Schroeter, Ber. 57B, 1990-2003] discloses a reversible isomerization reaction starting with either pure s-OHP or pure s-OHA isomer. In this reversible reaction, 50 percent yields of both s-OHP and s-OHA are obtained from either starting isomer in the presence of small amounts of AlCl.sub.3 at an isomerization temperature in a range of 70.degree. to 80.degree. C. In U.K. Pat. No. 694,961 s-OHP is isomerized to s-OHA in the presence of dispersed, finely-divided AlCl.sub.3 catalyst at an isomerization temperature in a range of 5.degree. to 45.degree. C. Yield of s-OHA ranged from about 70 to 83 weight percent with about 10 to 13 weight percent unidentified by-products. A 1978 West German publication [K. Handrick et al., "Production of Anthracene from Phenanthrene," Compend.-Dtsch. Ges. Kohlechem., 78-79(2), 1089-1106] describes a starting mixture containing s-OHP in the presence of about six weight percent s-OHA. After a 4-hour isomerization period conducted at room temperature, the reaction product mixture contains an equilibrium mixture of s-OHP and s-OHA isomers, there being a maximum of 64 weight percent s-OHA present. U.K. Pat. No. 2,065,698 to Handrick et al. describes isomerization of s-OHP in the presence of 3 to 6 weight percent AlCl.sub.3 catalyst and 15 to 60 weight percent methylene chloride solvent at a temperature of -30.degree. C. to +5.degree. C., which isomerization reaction after a 6- to 7-hour reaction period provides an overall yield of about 94% s-OHA isomer.
There is need, therefore, for s-OHP to s-OHA isomerization processes characterized by an increased yield of s-OHA and an increased rate of isomerization of s-OHP to s-OHA with low yield of by-product impurities.